Static eliminators are devices for producing both positive and negative ions in order to neutralize articles which have been charged to a particular polarity, usually as a result of electrostatic, electrical, frictional or mechanically created forces. When an A.C. voltage of fairly high magnitude is applied across the discharge points and the grounded casing or shield of such static bars, ions of both polarities are emitted.
In a shockless static bar, the A.C. power source is coupled to the discharge points by way of a capacitance in order to limit the maximum shorting current that can occur should an object, such as the fingers of operating personnel, accidentally shunt across the points and ground. Capacitively coupled bars are well known in the art and generally embody a construction in which a plurality of needle-like points are electrically connected to respective spaced apart conductive sleeves or plates which are arranged in close proximity to an elongated cable having an inner conductive core surrounded by a dielectric layer. A grounded housing or conductive member is adjacently spaced from the needle points and generally supports the discharge assembly while a high voltage A.C. source, in the range of about 2,500 to 15,000 volts, is connected across the inner conductive core and ground. The high voltage applied to the inner core produces an ionized field about the pointed ends of the discharge needles which has the effect, when directed toward a charged article, sheet or web, to neutralize the static charges accumulated thereon. Examples of various types of capacitively coupled static eliminators are shown and described in U.S. Pat. No. 2,163,294, No. 2,333,213, No. 3,120,626, No. 3,443,155, No. 3,585,448, No. 3,652,897, No. 3,875,461 and No. 4,092,543.
In any capacitively coupled static eliminator, the voltage applied to the points by a high voltage A.C. power source is a function of the relationship between the discharge electrode-to- high voltage conductor capacitance and the electrode-to-adjacent ground capacitance. That is, assuming that the point-to-ground resistance is extremely high, as would be the case, the voltage actually applied to the points with respect to the input voltage of the A.C. power source is, as may be seen from FIG. 2 of the drawings, determined by the following: ##EQU1## where V.sub.p =the voltage applied to each of the points,
C.sub.p =capacitance between the discharge electrodes and the high voltage cable conductor, PA1 C.sub.g =capacitance between discharge electrodes and adjacent ground, and PA1 V.sub.s =power supply input voltage.
As is apparent from the foregoing, a greater portion of the supply voltage, V.sub.s, can be applied to the points either by increasing C.sub.p or by decreasing C.sub.g. In view of the fact that an increase in C.sub.p necessarily augments the current discharge capability or shock, a result counter to the particular purpose of the capacitively coupled design, the present invention contemplates reduction in the value of C.sub.g. By minimizing C.sub.g, it is also readily apparent that the voltage on the points (V.sub.p) will approach the supply voltage V.sub.s whereby a much smaller supply voltage will produce the same degree of ionization at the points. Along with the reduction in power supply voltage requirements, there is also a lesser amount of insulation necessary in both the static bar and the power pack, thus enabling further diminution of their physical sizes as well as the components thereof, all with attendant economies in cost. Reduction in power supply voltage additionally reduces the likelihood of corona effects at locations other than the ionizing points themselves whereby objectionable ozone pollution of the environment by such static eliminators is minimized.
In the present invention, the diminution of the value of C.sub.g (discharge electrode-to-ground capacitance) is accomplished by providing a high voltage shield between the capacitor plates of such electrodes and the next adjacent ground while the points of such electrodes are fully exposed in juxtaposed spaced relation to ground in order to provide efficient ionization without the latter contributing appreciably to the ground capacitance C.sub.g because of the minute surface area of said points. The high voltage shield of this invention is effected by interposing a conductive member between ground and the capacitor plates which are capacitively coupled to the high voltage bus and connecting the high voltage power source or the high voltage bus to the shielding conductive member.
It is therefore an object of this invention to provide a shockless type static eliminator in which the power supply voltage requirements are reduced without sacrificing ionization efficiency.
Another object of this invention is to provide a capacitively coupled static eliminator in which the physical size of both the static bar and the power pack as well as all of the components thereof are diminished.
Still another object of this invention is to provide a capacitively coupled static eliminator in which the ground leg capacitance is reduced without increasing the capacitance of the high voltage leg, thereby retaining high ionization efficiency at limited current discharge capabilities.
Other objects of this invention are to provide an improved device of the character described which is easily and economically produced, sturdy in construction, and highly efficient and effective in operation.